The structure of powder snow avalanches

Sovilla³

2018

GEOphysical flow dynamics using pulsed Doppler radAR (GEODAR), a custom radar system, images avalanches over the entire slope with high spatial and temporal resolution at the experimental test site Vallée de la Sionne in Switzerland. Between winter seasons 2009/2010 and 2014/2015, data have been acquired from 77 avalanches. These data sets describe a wide variety of avalanches, which we classify in terms of seven flow regimes and combinations thereof. These flow regimes expand on previous classifications, with four identifiable dense flow regimes (where interaction between granules and with the flow bed dominates dynamics) and two different dilute flow regimes (where interaction between snow particles and the air becomes dominant). There is a further regime identified where snow balls simply roll down the mountain. A cold dense regime and a warm shear regime behave like noncohesive granular flows with velocity shear throughout the flow. A sliding slab regime and a warm plug regime occur when cohesion dominates and causes the flow units to act as solid‐like objects sliding on a thin shear zone. An intermittent regime connects the cold dense regime with the suspension regime and is characterized by highly fluctuating density and surging activity. GEODAR enables localization of these flow regimes and transitions between them in time and space. We discuss flow regime transitions in terms of snow properties, topography, speed, and size of the avalanches. This paper also serves as a reference for the data set which is made publicly available and should prove to be an invaluable resource for the development of physically based avalanche models.

Section: Introductionmentioning

confidence: 99%

GEODAR Data and the Flow Regimes of Snow Avalanches

Köhler

Sovilla³

2018

“…Figure 1b shows a schematic of a large mixed powder avalanche with the three main regions delineated. Sovilla et al [2015] showed direct evidence for an intermittent frontal region from density and impact pressure measurements. This region is characterized by a relatively low volume fraction with sporadic high-density clusters of snow ( Figure 1a and red vertical bars in Figure 1b).…”

Section: Introductionmentioning

confidence: 99%

The dynamics of surges in the 3 February 2015 avalanches in Vallée de la Sionne

Köhler¹,

Sovilla³

et al. 2016

Five avalanches were artificially released at the Vallée de la Sionne test site in the west of Switzerland on 3 February 2015 and recorded by the GEOphysical flow dynamics using pulsed Doppler radAR Mark 3 radar system. The radar beam penetrates the dilute powder cloud and measures reflections from the underlying denser avalanche features allowing the tracking of the flow at 111 Hz with 0.75 m downslope resolution. The data show that the avalanches contain many internal surges. The large or “major” surges originate from the secondary release of slabs. These slabs can each contain more mass than the initial release, and thus can greatly affect the flow dynamics, by unevenly distributing the mass. The small or “minor” surges appear to be a roll wave‐like instability, and these can greatly influence the front dynamics as they can repeatedly overtake the leading edge. We analyzed the friction acting on the fronts of minor surges using a Voellmy‐like, simple one‐dimensional model with frictional resistance and velocity‐squared drag. This model fits the data of the overall velocity, but it cannot capture the dynamics and especially the slowing of the minor surges, which requires dramatically varying effective friction. Our findings suggest that current avalanche models based on Voellmy‐like friction laws do not accurately describe the physics of the intermittent frontal region of large mixed avalanches. We suggest that these data can only be explained by changes in the snow surface, such as the entrainment of the upper snow layers and the smoothing by earlier flow fronts.

“…A powder snow avalanche (PSA), or mixed avalanche (Issler et al, ), is a type of avalanche that includes a region where some snow grains are suspended by turbulence, forming a powder cloud which covers dense flow regimes moving underneath (Sovilla et al, ). They are closely related to turbidity currents (Meiburg et al, ) and pyroclastic flows, and they are a type of gravity current (Simpson, ).…”

Section: Introductionmentioning

confidence: 99%

“…Upper panel: Schematic of a mixed powder snow avalanche entraining a generic layered snow cover (reproduced from Sovilla et al, ). Lettered regions correspond to: Int = the intermittency region; De = the basal dense layer; Su = the suspension layer; Sn = the static snow cover.…”

Section: Introductionmentioning

confidence: 99%

The Intermittency Regions of Powder Snow Avalanches

Sovilla¹,

Köhler³

2018

Powder snow avalanches are typically composed of several regions characterized by different flow regimes. These include a turbulent suspension cloud of fine particles, a dense basal flow, and an intermittency frontal region, which is characterized by large fluctuations in impact pressure, air pressure, velocity, and density, but whose origin remains unknown. In order to describe the physical processes governing the intermittency region, we present data from four large powder snow avalanches measured at the Vallée de la Sionne test site in Switzerland, which show that the intermittency is caused by mesoscale coherent structures. These structures have a length of 3–14 m and a height of 10 m or more. The structures can have velocities as much as 60% larger than the avalanche front speed and are characterized by an air/particle mixture whose average density can be as high as 20 kg/m3. This average density increases the drag on large granules by a factor of up to 20 compared to pure air, so that each structure can maintain denser snow clusters and single snow granules in suspension for several seconds. The intermittency region has importance for the dynamics of an avalanche, as it provides an efficient mechanism for moving snow from the dense layer to the powder cloud, but also for risk assessment, as it can cause large forces at large heights above the basal dense layer.